The invention relates to a tracking error signal generating circuit suitably used in an optical disk device, in which an RF signal is latched at a predetermined timing after it is subjected to A/D conversion to generate a digital tracking error signal.
FIG. 2A shows a servo byte pattern of a sample servo formed on an optical disk. An optical disk surface is divided into sectors, each sector of the optical disk containing, for example, forty-three (43) servo blocks. A servo block contains a two-byte servo portion and a 16-byte data portion subsequent thereto. Each byte has 15 bit positions. The two-byte servo portion is clearly shown in FIG. 2A. part (a). for four tracks. The first byte of the servo portion contains of two wobbled pits. The first wobbled pit is located at either the third or the fourth channel bit position. The second wobbled pit is located at the eighth channel bit position. The second byte of the servo portion contains one clock pit which is located at the twelfth channel bit position. The wobbled pits are arranged to the left and right of a track centerline.
When a pickup (a light spot for detecting information) accurately follows the track center, the intensities of reflected light from the left and right wobbled pits are equal to each other. When the pickup deviates to the left or right, away from the track center, the light intensities from the two wobbled pits change, depending on the direction and the amount of the deviation. As a result, based on the difference between the decreased values at two positions (or a level difference of RF signal, as shown in FIG. 2B. waveform (a)). a tracking error signal is produced and is held during a region of the data byte in which the tracking error signal continues.
The first wobbled pit in the first byte of the servo portion is changed every 16 tracks, from the third channel bit to the fourth channel bit, or vice versa. As a result, the interval between the two wobbled pits in the first servo byte in each servo portion is alternately longer and shorter every 16 tracks. Detecting the change in the interval makes it possible to count (within sixteen tracks) the number of the tracks even when data is searched at high speed.
The distance between the second wobbled pit (the eighth channel bit in the first byte of the servo portion) and the clock pit (the twelfth bit in the second byte of the servo portion) is set to be a special length D which is not used in the data portion. As a result, this distance D can be detected as a synchronizing signal. Various timing signals are generated based on the detected synchronization signal. The clock (FIG. 2B. waveform (b)) is generated in correspondence with the detected clock pit signal.
A mirror surface portion of the distance D is used as a focus sample area (FIG. 2A, waveform (b)). The focus error signal is sampled from that area and held for a period corresponding to the area of the following data byte. During the period corresponding to the passage of the data byte area past the pickup following the servo byte area, data is recorded by pits or with phase variation.
Because the conventional apparatus generates tracking error signals and the like utilizing an analog switch, a sample-and-hold circuit and the like, it is necessary to control the offset.